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Dark Matter Core Defies Explanation

  1. Mar 5, 2012 #1
  2. jcsd
  3. Mar 5, 2012 #2
    I agree this is interesting. I wonder if there is another way to confirm what we think we are seeing?

    I understand that DM was postulated to explain the distribution of rotation velocities of galaxies, so if these Galaxies had been stripped of some of their DM would it not also show up in the galaxy rotational distributions?
     
  4. Mar 5, 2012 #3
    You would think so. And confirming the observation would be great. It's a puzzler!
     
  5. Mar 6, 2012 #4

    Chalnoth

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    Well, the galaxies still do interact more than the dark matter, so it isn't completely absurd. It is interesting, and does speak to an interesting past history for this particular cluster. But the vast majority of clusters fall into the expected pattern, with dark matter and galaxies well-aligned.
     
  6. Mar 6, 2012 #5

    Bobbywhy

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    Here is an excerpt from NASA:

    "The blend of blue and green in the center of the image reveals that a clump of dark matter resides near most of the hot gas, where very few galaxies are found. This finding confirms previous observations of a dark-matter core in the cluster. The result could present a challenge to basic theories of dark matter, which predict that galaxies should be anchored to dark matter, even during the shock of a collision."

    http://www.nasa.gov/multimedia/imagegallery/image_feature_2189.html
     
    Last edited: Mar 6, 2012
  7. Mar 6, 2012 #6
    The Bullet Cluster is claimed to provide the best evidence to date for the existence of dark matter (see for instance wikipedia entry). Is this new observation the best evidence to date against the existence of dark matter, then?
     
  8. Mar 6, 2012 #7

    Chalnoth

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    Not really. As I said, the vast, vast majority of clusters are very easy to understand in terms of dark matter. The bullet cluster is also easy to understand largely because it's easy to see what happened to separate the cluster gas from the dark matter. That a few clusters are difficult to understand in terms of dark matter is expected.

    My bet is that if a group of scientists are really interested in this, they'll find a reasonable solution within a few years. If they haven't already (I haven't looked, sorry).
     
  9. Mar 6, 2012 #8
    Hmm, we are talking about clusters colliding here, do you know any good example besides the Bullet cluster and more recently MACSJ0025?
     
  10. Mar 6, 2012 #9

    Chalnoth

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  11. Mar 6, 2012 #10
    Maybe this is evidence for the "Mirror Matter" dark matter hypothesis.

    http://en.wikipedia.org/wiki/Mirror_matter

    Mirror matter is hypothesized to be just like ordinary matter but is "right-handed" rather than "left-handed".

    Perhaps most mirror matter is in the form of mirror Hydrogen and mirror Helium which forms clouds around galaxies whereas some mirror matter might be in the form of heavier mirror elements that can clump together.
     
    Last edited: Mar 6, 2012
  12. Mar 6, 2012 #11
  13. Mar 6, 2012 #12

    Chalnoth

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    I don't quite see how, but I should mention that mirror matter is basically just one of many dark matter hypotheses. Mirror matter must be pretty different from normal matter in order to explain our observations, as dark matter interacts very weakly both with itself and with normal matter.
     
  14. Mar 6, 2012 #13
    If mirror matter exists and is mostly in the form of cold mirror hydrogen/helium surrounding galaxies then I suppose it would not interact strongly with itself if those galaxies collide.

    Is that correct?
     
  15. Mar 6, 2012 #14

    Chalnoth

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    Our best constraints on the interaction of dark matter with itself and with normal matter comes from the early universe, where all of the normal matter was an ionized plasma. At that time, the dark matter still didn't interact much at all with itself or with normal matter, so it can't simply be made of the same stuff as normal matter is.
     
  16. Mar 6, 2012 #15
    Ok - so if dark matter was mirror matter then in the early Universe it should have been a mirror ionized plasma in the same way that normal matter was an ionized plasma. Thus even if it didn't interact with normal matter at all it should still have interacted strongly with itself. We don't see evidence of that so dark matter is probably not mirror matter.

    Thanks for the clarification.
     
  17. Mar 6, 2012 #16

    Chalnoth

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    Well, at least not precisely. It basically has to interact with itself considerably more weakly than normal matter interacts with itself. So it can't be an exact mirror.

    What might also help is if the physics causes recombination (when the universe first turns from a plasma into a gas) to occur sooner with mirror matter than with normal matter. Though I think it would still have to interact more weakly for us to not see the interactions.
     
  18. Mar 6, 2012 #17

    marcus

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    Dearly Missed

    I don't think anyone has given the link to the journal article yet
    http://arxiv.org/abs/1202.6368
    A Study of the Dark Core in A520 with Hubble Space Telescope: The Mystery Deepens
    M. J. Jee, A. Mahdavi, H. Hoekstra, A. Babul, J. J. Dalcanton, P. Carroll, P. Capak
    (Submitted on 28 Feb 2012)
    We present a Hubble Space Telescope/Wide Field Planetary Camera 2 weak-lensing study of A520, where a previous analysis of ground-based data suggested the presence of a dark mass concentration. We map the complex mass structure in much greater detail leveraging more than a factor of three increase in the number density of source galaxies available for lensing analysis. The "dark core" that is coincident with the X-ray gas peak, but not with any stellar luminosity peak is now detected with more than 10 sigma significance. The ~1.5 Mpc filamentary structure elongated in the NE-SW direction is also clearly visible. Taken at face value, the comparison among the centroids of dark matter, intracluster medium, and galaxy luminosity is at odds with what has been observed in other merging clusters with a similar geometric configuration. To date, the most remarkable counter-example might be the Bullet Cluster, which shows a distinct bow-shock feature as in A520, but no significant weak-lensing mass concentration around the X-ray gas. With the most up-to-date data, we consider several possible explanations that might lead to the detection of this peculiar feature in A520. However, we conclude that none of these scenarios can be singled out yet as the definite explanation for this puzzle.
    Published in Astrophysical Journal

    The alternative scenarios being considered to explain the dark core are described on page 7.

    Figures 4 and 5 show contour density maps and are graphically clear and detailed. Better than what appeared in the news article. It's worth visiting the professional journal article if only to get a look at these contour maps.
     
    Last edited: Mar 6, 2012
  19. Mar 6, 2012 #18
  20. Mar 6, 2012 #19
    Nope, time ago was shown that dark matter fails for the bullet cluster

    Moreover, the past year solid evidence against dark matter was already provided

    http://www.bbc.co.uk/news/science-environment-12571965

    http://physics.aps.org/articles/v4/23
     
  21. Mar 6, 2012 #20

    marcus

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    amusing quip, but I don't think it's right. Density contour maps of DM clouds in many locations (e.g. around clusters of galaxies) have been made. The maps are interesting, show differences, show different things happening, and require explanation.

    the hypothetical planet "Vulcan" was not something observed to be explained,
    it was a KLUDGE explanation of precession in Mercury's orbit.
    http://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet)
    an unexplained peculiarity in Mercury's orbit was established as early as 1860, I think.

    DM is also an essential feature of simulations of filamentary structure formation in early universe---and the simulations yield realistic-looking results.

    DM is just too economical a way to explain this bunch of different things going on at widely different scales---galaxy, clusters of galaxies, filamentary structure.
    The nature of DM is a major puzzle. It's not a one-time kludge like the hypothetical planet Vulcan.

    Maybe you were kidding. Nice epigram

    BTW I also think it's pretty clear that our present law of gravity, GR, is going to be MODIFIED. It is deficient in some recognized ways and people are working on that. So if you are trying to say GR has to be modified, you are "preaching to the converted". That, I think, is a different problem. Modifying, say in the course of developing a quantum theory of geometry and matter, is unlikely to supplant DM in all its different explanatory roles.
     
    Last edited: Mar 6, 2012
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